The present invention is directed to bicycle crank spindles and, more particularly, to a hollow crank spindle for a bicycle.
A bicycle is equipped with crank spindles, hub spindles, and other spindles. Ideally, these spindles should have minimum weight because motive power is provided by human effort. A known technique of weight reduction is to use hollow crank spindles in place of solid ones (e.g, Japanese Laid-Open Utility Model Application 52-60058). In a crank spindle, constant torsional stress and other loads are applied because the crank receives the tread force of the left pedal, captures this force as a torsional torque, and transmits this torque as motive power to the chain via the front chainwheel. This is especially true in cotterless-type cranks wherein the two ends of the spindle have a rectangular cross section to nonrotatably fix the crank arms to the spindle. Unfortunately, when a hollow crank spindle is adopted, increasing the inside diameter of the hollow portion reduces the wall thickness of the corresponding part of the spindle. This, in turn, reduces the strength of the spindle. In order to provide sufficient weight reduction while maintaining strength, it is desirable to increase the outer diameter of the spindle. In addition, the inner bore of a hollow crank spindle should be maximized because of the limitations imposed by boring tools.
The Applicant has already proposed a structure in which the crank spindle and the left and right cranks are joined by serrations formed on the outer peripheral surface of the crank spindle in order to enhance the strength of the joints. This structure is disclosed in Japanese Patent Application 8-46657. The bonding strength of the crank attachments of the hollow crank spindle can be enhanced by increasing the diameter of these attachments.
Unfortunately, the bore diameters of the bottom brackets for supporting crank spindles are standardized. Also, because a crank spindle is supported by rolling elements in the inner bore of the bottom bracket, gaps are needed for the rolling elements and for the fixed cup and the adjusting cup that support the rolling elements. Thus, the diameter of the middle portion of the hollow crank spindles supported in the bottom bracket cannot be increased arbitrarily.